Method for controlling the braking system of a motor vehicle

ABSTRACT

A method is used to control a brake system ( 1 ) including an electrically controllable generator ( 4 ) and a number of friction brakes ( 2 ) in such a fashion that high braking comfort can be achieved by means of the brake system ( 1 ). To this end, the total deceleration consists of a parallel configuration of deceleration components of the friction brakes ( 2 ) and the generator ( 4 ), the nominal brake torque of the generator ( 4 ) is determined by way of determining a total nominal deceleration and the generator ( 4 ) is controlled by means of this nominal brake torque.

BACKGROUND OF THE INVENTION

The present invention relates to a method for controlling a brake systemof a motor vehicle, in particular for controlling a regenerative brakesystem with a number of friction brakes and an electric generator for amotor vehicle. It further relates to a software module for controllingthe method.

The purpose of regenerative brake systems in motor vehicles involvesstoring at least part of the energy produced during braking in thevehicle and re-using it for the drive of the vehicle. This allowsreducing the energy consumption of the vehicle in total, increasing theefficiency, and thus rendering its operation more economical. To thiseffect, motor vehicles with a regenerative brake system generallyinclude different types of brakes, which are also called brakeactuators.

Typically, hydraulic friction brakes, as known from customary motorvehicles, and an electro-regenerative brake are employed in thisarrangement. Like in conventional friction brakes, the brake pressurefor the friction brakes is generated by means of a brake pressuregenerating means or by way of the brake pedal movement, respectively.The electro-regenerative brake is generally configured as an electricgenerator, through which at least part of the total brake output isgenerated. The produced electric energy is fed into a storage mediumsuch as an on-board battery, or fed back, respectively, and is reusedfor the drive of the motor vehicle by way of an appropriate drive.

Regenerative brake systems can be designed as so-called serialregenerative concepts where the component of the brake torque which isproduced by the generator is as high as possible. In contrast thereto,parallel or so-called residual-moment-based regenerative concepts areknown, where the brake torque is distributed in predefined ratios to thebrake actuators. In parallel brake systems of this type, typically,there exist deceleration components of both the generator and thefriction brakes in all operating conditions. Mixed concepts of these twobrake concepts are also known. It is common to all systems that severalbrake actuators are used for simultaneous braking at least in someranges of the brake torque to be generated so that the totaldeceleration is composed of the deceleration components of the brakeactuators.

So-called ‘brake-by-wire’ brake systems are principally known in serialregenerative brake systems. In ‘brake-by-wire’ brake systems, thebraking energy is generally split up into components of the frictionbrakes and into components of the electric generator, said split-upbeing dependent on the nominal brake torque, the charging condition ofthe battery, and especially the field of operation and other specialproperties of the generator. Due to the split-up of braking energy, thebrake pressure is built up independently of the hydraulic influence ofthe brake pedal in ‘brake-by-wire’ brake systems.

In conventional brake systems, however, which have only one frictionbrake, the brake pressure is built up depending on the position of thebrake pedal. In this arrangement, the pressure of a braking medium isbuilt up by way of the position of the brake pedal with or withoutauxiliary energy, the braking medium being received by the frictionbrake. Thus, the pedal position corresponds to the braking behavior ofthe motor vehicle. Exceptions can be the use of electronic safetysystems such as the electric stability program (ESP), which can comprisedevices for the independent brake pressure development irrespective ofthe brake pedal position.

Compared to a conventional brake system, a ‘brake-by-wire’ brake systementails high effort and structure and costs.

SUMMARY OF THE INVENTION

In view of the above, an object of the invention is to disclose a methodfor controlling a brake system which includes an electricallycontrollable generator and a number of friction brakes so that highbraking comfort can be achieved with a simple brake system. Anotherobjective is to disclose a software module which is especiallyappropriate for implementing the method.

With respect to the method, this object is achieved by the inventionbecause the total deceleration is composed of a parallel configurationof deceleration components of the friction brakes and the generator, andthe nominal brake torque of the generator is determined by way ofdetermining a total nominal deceleration, and the generator iscontrolled by way of this nominal brake torque.

The invention is based on the reflection that an acceptable pedal feeldevelops in a parallel braking method or brake system wherein in abraking operation the brake torque is composed of a parallelconfiguration of a generator brake torque and the brake torque offriction brakes. In this arrangement, the method shall prevent an activebraking medium pressure so that there is no reactive effect on the brakepedal. Therefore, a braking request is converted into hydraulic brakingmedium pressure of the friction brakes, and the generator torque issuperimposed on the braking torque, which results from an adaptation tothe braking request. The position of the brake pedal corresponds to thebraking performance of the motor vehicle because the total decelerationcorresponds to the brake pressure of the hydraulic friction brake. Thepressure of the friction brake, in turn, corresponds to the position ofthe brake pedal because a correspondingly higher amount of hydraulicfluid is urged from the brake pedal to the friction brakes e.g. when thebrake torque is increased. The total nominal deceleration is determinedfor controlling the generator by way of a corresponding nominal braketorque. Subsequently, the nominal deceleration component of thegenerator can be determined by way of the deceleration component of thefriction brakes, and the generator can be controlled on the basisthereof or, respectively, on the basis of the resulting nominal braketorque of the generator.

In order to demand the generator brake torque in particular only in itsworking range and not to overload the generator, the brake torque of thegenerator is favorably limited by a maximum brake torque in a selectedspeed range. Besides, it is thereby ensured that the brake torquecomponent of the friction brakes in this speed range is still sufficientso that the brake pedal position corresponds in the best way possible tothe braking performance of the motor vehicle.

Suitably, the range between 10 and 50 km/h is chosen as the speed range.Preferably, that torque is predefined as the maximum brake torque forthe generator which corresponds to a contribution of the generator tothe total deceleration of roughly 0.15 g. In general, the torque of anelectric generator rises steeply at low speeds, especially between 10and 20 km/h. In a brake torque limitation starting from 10 km/h, thebrake torque of the generator is smoothed in a transition of the pointof abrupt steep rise.

For a floating transition of the brake torque control of the generatorat the fringes of the speed range, abrupt brake torque changes of thegenerator at the fringe of the speed ranges are preferably smoothed inorder to avoid a negative reaction to the braking performance.

Advantageously, the generator is operated to be acting as soon as thereis no positive acceleration of the motor vehicle. The driver thereforefeels the deceleration effect of the generator as an additional enginebrake, while braking by the friction of the engine can be performed inaddition.

Preferably that torque is preset to the generator as an appropriatebrake torque, which corresponds to a contribution of the generator tothe total deceleration of approximately 0.1 g.

To determine the total nominal deceleration, this deceleration issuitably determined by means of a travel indicator of the brake pedal ofthe brake system. To determine the brake pressure which prevails at thebrake pedal in a defined brake pedal position and can also be used fordetermining the total nominal brake torque, said brake pressure isdetermined preferably by way of a pressure sensor that is positioned ata hydraulic line of the brake system leading to a friction brake.

Still further signals can be used for determining the total nominalbrake torque. Favorably, signals being available in the brake system areused. In this arrangement, signals are used in particular which are usedfor controlling safety systems such as ABS (anti-lock system), ESP(Electronic Stability Program), or ASC (anti-slip control) and areprovided in the brake system.

For an appropriate control and an implementation of the method into abrake system, a brake system is appropriately controlled by way of asoftware module using a method described above. To this end, anelectrically controlled brake system can also be extended by a softwaremodule of this type.

Preferably, a software module is a component part of a brake system of amotor vehicle.

The advantages achieved with the invention especially include thepossibility of controlling a regenerative brake system in such a fashionthat reasonable braking comfort is achieved, with the decelerationbehavior of the motor vehicle corresponding to the position of the brakepedal. It is another advantage of the invention that the effort involvedfor realizing the brake system described is comparatively low. Thus,only an appropriate software module is required to control the brakesystem. Therefore, the brake system controlled by a software module hasa very favorable cost-benefit ratio.

The comparatively low expenditure in additional brake system componentswhich are required for controlling the brake system, and simple controloperations also permit a high degree of reliability of the brake system.

It is another advantage that the software module can be retrofitted toelectrically controllable brake systems. Both regenerative brake systemsand conventional brake systems with friction brakes and an additionalelectric generator are feasible.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment is explained by way of FIGS. 1 to 2. In the drawings:

FIG. 1 shows a basic circuit diagram of a brake system (1), and;

FIG. 2 shows a brake operation, with proportionate brake torques ofgenerator (4) and friction brakes (2) as a function of time.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a basic circuit diagram of a brake system 1 for amotor vehicle. Brake system 1 is a regenerative brake system 1 which,apart from four friction brakes 2, also includes an electric generator 4for producing electric energy. The brake pressure required for thefriction brakes 2 is produced by way of the brake pedal 6 and the brakebooster 7 with the master brake cylinder 8. As this occurs, the masterbrake cylinder 8 conducts braking medium B through the hydraulic lines10 to the friction brakes 2. The master brake cylinder 2 is configuredas a tandem master cylinder to achieve an appropriate delivery rate. Thebrake system 1 is designed to provide great braking comfort because themotion of brake pedal 6 corresponds to the braking performance of themotor vehicle. To this effect, the brake system 1 is designed as aparallel regenerative brake system 1. The total deceleration of themotor vehicle in a braking operation is composed of decelerationcomponents of generator 4 and friction brakes 2. To correspondinglyactuate the generator 4, the brake system 1 includes a software module12 presetting a nominal brake torque to the generator 4. The latterbrake torque is determined from the total nominal deceleration and thedeceleration component of the friction brakes 2 with the software module12. The brake pedal travel or the position of the brake pedal 6,respectively, and the braking medium pressure of the braking medium B inthe hydraulic lines 10 are taken into account as input quantities. Thebrake pedal travel is determined by means of the travel indicator 24which is positioned at the brake pedal 6, while the braking mediumpressure is measured at the hydraulic line 10 by means of pressuresensor 26.

The control of the brake system 1 is explained in more detail in thelight of a braking operation. Therefore, a braking operation withproportionate brake torques of generator 4 and friction brakes 2 isshown in FIG. 2 as a function of time. An initial speed of 100 km/h ismade the basis in the braking operation. Curve 1 represents the brakingrequest or the total nominal deceleration, respectively, which thedriver introduces by way of the brake pedal 6. In this arrangement, thenominal deceleration of the brake at first rises linearly when the brakepedal 6 is depressed, stays constant during the braking operation, andis reduced again linearly to zero at the end by releasing the brakepedal 6.

Before the braking operation commences, the motor vehicle, in theabsence of acceleration, is already decelerated by the generator 4 usingthe brake torque that corresponds to a deceleration of the motor vehicleof 0.1 g, which the driver feels to be an engine brake. Thisdeceleration is not shown FIG. 2.

When the brake pedal 6 is depressed, both the brake torque of thegenerator 4 and that of the friction brakes 2 will rise linearly, andthe brake torque component of the generator 4 amounts to 40 % (curve 2)and that of the friction brakes amounts to 60 % (curve 3) when themaximum total deceleration is reached.

The available brake torque of generator 4 will rise when 60 km/h arereached. However, the software module will limit the brake torque to abrake torque which corresponds to a proportionate deceleration of themotor vehicle by way of the generator 4 of roughly 0.15 g. The rise upto this maximum brake torque of the generator 4 to a component of 50 %of the total brake torque is controlled to be rising linearly.Simultaneously, the brake torque component of the friction brakeslikewise drops to 50 %. When the brake pedal 6 is released, the braketorque components of both the friction brakes and the generator arereduced linearly to zero.

List of Reference Numerals:

-   1 brake system-   2 friction brake-   4 generator-   5 brake pedal-   7 brake booster-   8 master brake cylinder-   10 hydraulic line-   12 software module-   24 travel indicator-   26 pressure sensor-   B braking medium

1-12. (canceled)
 13. A method for controlling a brake system of a motorvehicle, the motor vehicle includes a parallel configuration of anelectrically controllable generator and a number of hydraulic frictionbrakes, the method comprising: determining a total nominal deceleration;determining a nominal brake torque of the generator based on thedetermined total nominal deceleration; and controlling the generatorbased on the nominal brake torque.
 14. The method according to claim 13,wherein the brake torque of the generator is limited by a maximum braketorque in a selected speed range.
 15. The method according to claim 14,wherein a range between 10 and 50 km/h is chosen as the speed range. 16.The method according to claim 14, wherein at the maximum brake torquefor the generator (4) a torque is predetermined that corresponds to acontribution of the generator (4) to a total deceleration of roughly0.15 g.
 17. The method according to claim 14, wherein abrupt braketorque changes of the generator at fringes of the speed range aresmoothed.
 18. The method according to claim 13, wherein the generator isoperated as soon as there is no positive acceleration of the motorvehicle.
 19. The method according to claim 18, wherein in the absence ofpositive acceleration of the motor vehicle, the generator torque ispreset to correspond to a contribution of the generator (4) to the totaldeceleration of approximately 0.1 g.
 20. The method according to claim13, wherein a total nominal deceleration is determined by way of atravel indicator of the brake pedal of the brake system.
 21. The methodaccording to claim 13, wherein a total nominal deceleration isdetermined by way of a pressure sensor which is positioned at ahydraulic line leading to a friction brake.
 22. The method according toclaim 13, wherein signals are used to determine the total nominaldeceleration which are available to the brake system.
 23. A softwaremodule for controlling a regenerative brake system of a motor vehicle,the motor vehicle includes a parallel configuration of an electricallycontrollable generator and a number of hydraulic friction brakes, thesoftware module comprising: code for determining a total nominaldeceleration; code for determining a nominal brake torque of thegenerator based on the determined total nominal deceleration; and codefor controlling the generator based on the nominal brake torque.